This invention relates, in general, to manufacturing semiconductor devices and, more particularly, to a method of making airbridge metal interconnects on semiconductor devices.
Airbridge metal interconnects are typically employed in gallium arsenide devices, such as microwave integrated circuits. Gallium arsenide devices are used in high frequency applications. The performance of high frequency integrated circuits is significantly reduced if high parasitic capacitances are present. Airbridge technology reduces parasitic capacitances, thus permits high frequency integrated circuits, greater than 1 GHz, to function with lower loss and a higher gain than those fabricated with non-airbridge technologies. An airbridge metal interconnect is one in which the typical dielectric layer is replaced by air. Using nitride or oxide as a dielectric layer is not acceptable in high frequency devices because the high parasitic capacitances of nitride or oxide result in unacceptable high loss and low gain.
In the past, airbridges have been fabricated by applying a bottom photoresist layer to the surface of the device structure. Subsequently, the bottom photoresist layer is patterned and a thin metal ground plane is evaporated onto the surface. Another layer of photoresist is applied and patterned over the metal ground plane A thick layer of metal is then electroplated in the openings of the top photoresist. A problem then arises when removing the top photoresist layer; the developer or solvent used to remove the top photoresist layer also dissolves the bottom photoresist layer through the sides of the structure and through the thin metal ground plane. The ground plane metal, typically comprised of titanium and gold, is then removed with hydrofluoric acid and a cyanide based acid called Technistrip. Hydrofluoric acid is an etchant for titanium, but it also etches nitride, thus any nitride on the underlying device surface is also etched if there are regions in the bottom photoresist layer which have been dissolved. The cyanide based acid etches gold, thus other gold on the underlying device structure is also etched. This will result in poor device yields. Thus, there is a need to provide a method of forming airbridge metal interconnects where the underlying semiconductor device structure is not disturbed.
One such method involves using an ion-miller process to remove the top photoresist layer and ground plane metal. The ionic milling etches only in the vertical direction, thus does not attack the bottom photoresist through the sides. Using an ion-miller process is effective, however, is more expensive than using solvents and wet etches to remove the top photoresist layer and the metal ground plane. In addition, an ion-miller process can potentially leave metal residue over the wafer. Other methods of fabricating airbridge metal interconnects use a photoresist lift-off technique. Lift-off techniques, however, do not have good process latitude for thick metal. Thus, methods other than lift-off techniques are desirable to increase yield.
By now, it should be appreciated that it would be advantageous to provide a method of fabricating airbridge metal interconnects with an improved process yield.
Accordingly, it is an object of the present invention to provide an airbridge metal interconnect process which produces stable airbridge structures.
Another object of the present invention is to provide a method of fabricating airbridge interconnects without etching of the underlying device structures.
An additional object of the present invention is to provide an improved method of fabricating airbridge interconnects which has good process latitude, thereby leading to higher device yields and the ability to make high frequency devices.